In the electrophotographic reproducing arts, it is necessary to deposit a uniform electrostatic charge on an imaging surface, which charge is subsequently selectively dissipated by exposure to an information containing optical image to form an electrostatic latent image. The electrostatic latent image may then be developed and the developed image transferred to a support surface to form a final copy of the original document.
In addition to precharging the imaging surface of a xerographic system prior to exposure, corona devices are used to perform a variety of other functions in the xerographic process. For example, corona devices aid in the transfer of an electrostatic toner image from a reusable photoreceptor to a transfer member, the tacking and detacking of paper to the imaging member, the conditioning of the imaging surface prior to, during, and after the deposition of toner thereon to improve the quality of the xerographic copy produced thereby.
Both D.C. and A.C. type corona devices are used to perform many of the above functions.
The conventional form of corona discharge device for use in reproduction systems of the above type is shown generally in U.S. Pat. No. 2,836,725 in which a conductive corona electrode in the form of an elongated wire is connected to a corona generating D.C. voltage. The wire is partially surrounded by a conductive shield which is usually electrically grounded. The surface to be charged is spaced from the wire on the side opposite the shield and is mounted on a grounded substrate. Alternately, a corona device of the above type may be biased in a manner taught in U.S. Pat. No. 2,879,395 wherein an A.C. corona generating potential is applied to the conductive wire electrode and a D.C. potential is applied to the conductive shield partially surrounding the electrode to regulate the flow of ions from the electrode to the surface to be charged. Other biasing arrangements are known in the prior art and will not be discussed in great detail herein.
There have been charging devices that display some of the qualities of both a corotron and a scorotron. A reference screen or series of wires each having a relatively large opening to allow the coronode to initially "see" the receiver unobscured, has also been used. This opening provides the corotron feature and the wire screen provides the scorotron feature. Generally, these devices operate with grounded shields and therefore are quite inefficient.
Various approaches to answering these problems have been tried in the past. For example, U.S. Pat. No. 3,717,801 discloses a shieldless corona device having a plurality of coronodes which together discharge a corona current onto a conductive receiving surface. A multiple coronode corona device uses a ground plane mounted underneath the receiving medium to absorb and dissipate any wayward charging effects. U.S. Pat. No. 3,937,960 is directed to a corona charging device for an electrophotographic reproduction machine which uses a flexible conductive control plate, adjustably mounted above a coronode discharging wire, to vary the electric field strength generated by corona discharging electrode during charging of conductive surface. U.S. Pat. No. 3,711,710 discloses a corona charging system wherein a corona point electrode, protruding from a planar conductive shield plate, delivers a discharging current to an electrographic recording member. An adjustably mounted conductive shield plate not only shields the discharge of a corona point electrode, but also controls the charge accumulated on a dielectric surface by translating a vertical degree of freedom about the long corona electrode. U.S. Pat. No. 3,813,547 shows a corona generating device comprising a coronode which discharges an ion current flow onto a photoreceptror surface. An ion current flow is regulated and shielded by a single conductive substrate which is mounted above a coronode wire. U.S. Pat. Nos. 4,700,261 and 4,785,372 disclose dual electrode corona charging devices which together project an ion flow onto a charge receiving surface. Although these attempts at solving the above-mentioned charging problem have had some success, they have not been entirely satisfactory.